1. Field of the invention
The present invention relates to a glazing panel having solar screening properties. The panel according to one aspect of the invention takes the form of a substrate carrying a spray-formed pyrolytic coating containing tin and antimony. According to another aspect of the invention the invention takes the form of a substrate carrying a coating containing tin and antimony formed by chemical vapor deposition.
Reflective transparent solar control glazing panels have become a useful material for architects to use for the exterior facade of buildings. Such panels have aesthetic qualities in reflecting the immediate environment and, being available in a number of colors, in providing a design opportunity. Such panels also have technical advantages by providing the occupants of a building with protection against solar radiation by reflection and/or absorption and eliminating the dazzling effects of intense sunshine, giving an effective screen against glare, enhancing visual comfort and reducing eye fatigue.
From a technical point of view, it is desired that the glazing panel shall not pass too great a proportion of total incident solar radiation in order that the interior of the building shall not become overheated in sunny weather. The transmission of total incident solar radiation may be expressed in terms of the “solar factor”. As used herein. the term “solar factor” means the sum of the total energy directly transmitted and the energy which is absorbed and re-radiated on the side away from the energy source, as a proportion of the total radiant energy incident of the coated glass.
Reflective transparent solar control glazing has also become much used in vehicle windows, where the objective is to protect the vehicle occupants against solar radiation. It has been used in railway carriages as side windows and in road vehicles for side, rear and roof windows. It has further been proposed to form the whole roof area of a motor car. It serves to provide protection against solar radiation by reflection and/or absorption and by eliminating the dazzling effects of intense sunshine, thereby giving an effective screen against glare, enhancing visual comfort and reducing eye fatigue. In this case the main energy factor to be considered is the total energy directly transmitted (TE), since the energy which is internally absorbed and re-radiated (AE) is dissipated by the movement of the vehicle. The essential aim of the vehicle panel is thus to have a low TE factor.
The properties of the coated substrate discussed herein are based on the standard definitions of the International Commission on Illumination—Commission Internationale de Z'Eclairage (“CIE”).                The standard illuminants quoted herein are CIE Illiuninant C and Illuminant A. Illuminant C (mostly used for evaluating the optical properties of glazing panels for buildings) represents average daylight having a color temperature of 6700° K. Illuminant A (which equates to the light emitted by car headlamps and is therefore generally used to evaluate the optical properties of glazing panels for motor vehicles) represents the radiation of a Planck radiator at a temperature of about 2856° K.        The “luminous transmittance” (TL) is the luminous flux transmitted through a substrate as a percentage of the incident luminous flux.        The “luminous reflectance” (RL) is the luminous flux reflected from a substrate as a percentage of the incident luminous flux.        The “energy transmission” (TE) is the total radiant energy directly transmitted through a substrate as a percentage of the incident radiant energy.        The “energy reflection” (RE) is the radiant energy reflected from a substrate as a percentage of the incident radiant energy.        The “solar factor” (FS) is the ratio of the sum of the total energy directly transmitted through a substrate (TE) and the energy which is absorbed and re-radiated on the side away from the energy source (AE) as a proportion of the total radiant energy incident on the substrate.        The “selectivity” of the coated substrate relates to the balance between luminous transmission and energy transmission. In the case of building glass it is often defined as the ratio of the luminous transmittance to the solar factor (TL/FS), but for vehicle glass it commonly refers to the ratio of the luminous transmittance to the energy transmission (TL/TE).        The “dominant wavelength” (1D) is the peak wavelength in the range transmitted or reflected by the coated substrate.        The “purity” (p) of the color of the substrate refers to the excitation purity measured with Illuminant C. It is specified according to a linear scale on which a defined white light source has a purity of zero and the pure color has a purity of 100%. The purity of the coated substrate is measured from the side opposite the coated side.        The “emissivity” (e) is the ratio of the energy emitted by a given surface at a given temperature to that of a perfect emitter (black body with emissivity of 1.0) at the same temperature.        The term “refractive index” (n) is defined in the CIE International Lighting Vocabulary, 1987, page 138.        
2. Description of the Related Art
A number of techniques are known for forming coatings on a vitreous substrate, including pyrolysis. Pyrolysis generally has the advantage of producing a hard coating, which precludes the need for a protective layer. The coatings formed by pyrolysis have durable abrasive- and corrosion-resistant properties. It is believed that this is due in particular to the fact the process involves depositing of coating material onto a substrate which is hot. Pyrolysis is also generally cheaper than alternative coating processes such as sputtering, particularly in terms of the investment in plant. The deposit of coatings by other processes, for example by sputtering, led to products with very different properties, in particular a lower resistance to abrasion and occasionally a different refractive index.
A wide variety of coating materials have been proposed for glazing panels, and for several different desired properties of the glazing. Tin oxide, SnO2, has been widely used, often in combination with other materials such as other metal oxides. GB Patent 1455148 teaches a method for pyrolytically forming a coating of one or more oxides on a substrate, primarily by spraying compounds of a metal or silicon, so as to modify the light transmission and/or light reflection of the substrate, or to impart antistatic or electrically conductive properties. Its examples of specified oxides include ZrO2, SnO2, Sb2 O3, TiO2, Co3O4, Cr2O3, SiO2 and mixtures thereof. Tin oxide (SnO2) is seen advantageous because of its hardness and its ability to have antistatic or electrically conductive properties. GB Patent 2078213 relates to a sequential spray method for pyrolytically forming a coating on a vitreous support and is particularly concerned with tin oxide or indium oxide as the main coating constituents. When its metal coating precursor is tin chloride this is advantageously doped with a precursor selected from ammonium bifluoride and antimony chloride in order to increase the electrical conductivity of the coating.
It is also known that where a coating of tin oxide is formed by pyrolysis of SnCl4, the presence of a dopant such as antimony chloride SbCl5, directly mixed with the tin chloride SnCl4, improves the absorption and reflection of some near solar infrared radiation.
Our earlier Patent, GB 2200139, describes and claims a method of forming a pyrolytic tin oxide coating on a hot glass substrate by spraying a solution containing a tin compound and additives which produce in the coating both fluorine and such materials as antimony, arsenic, vanadium, cobalt, zinc, cadmium, tungsten, tellurium and manganese so as to give the coating a low emissivity and a low specific internal haze factor. While the resultant coating has many desirable properties it falls short of the combination of properties now being sought.